The present invention relates generally to a reticle, mask or photomask used in photolithography for the manufacture of semiconductor devices, and more particularly to a reticle with reduced thermally induced distortion.
In the manufacture of semiconductors using photolithography, a mask or reticle is used to project the image of a circuit pattern onto a photosensitive resist covered wafer. As the feature size of the elements in the circuit pattern become smaller, the need for increased resolution and dimensional accuracy also increase. There have been many advances in improving the mask or reticle used in the photolithographic process. One such improvement is disclosed in U.S. Pat. No. 5,322,748 entitled Photomask And Method Of Manufacturing Thereof Comprising Trapezoidal Shaped Light Blockers Covered By A Transparent Layer, issuing to Watakabe et al on Jun. 21, 1994, which is herein incorporated by reference. Therein disclosed is a phase shift mask used to improve resolution of the image projected onto a photosensitive wafer. While such improvements have advanced the state of the art, this invention does not deal with this type of improvement. As the throughput of the tools used to manufacture semiconductor wafers is increased due to the need to increase production, problems may arise. For example, in an effort to decrease the exposure time in a photolithographic process, high energy flux illumination is used. As a result of this increase in intensity or flux, portions of the mask or reticle may absorb sufficient amounts of this energy to create undesirable thermal distortions. These thermal distortions will create errors in the imaging of the pattern contained on the reticle or mask which is projected onto a photosensitive resist covered wafer. Accordingly, lower yields would be expected in the finished semiconductor device, thereby increasing the cost of manufacture. Accordingly, there is a need for a mask or reticle that has reduced thermal distortion which can accommodate the higher energy flux illumination used in an effort to increase productivity, and as the operating resolution diminishes these effects will become more significant.
The present invention is directed to a reticle or mask used in photolithography. A transparent substrate has a reflective or dielectric layer placed thereon. An opaque or blocking layer is placed on top of the reflective or dielectric layer. Portions of the opaque or blocking layer and the reflective or dielectric layer are removed to form the desired circuit pattern to be projected onto a photosensitive resist covered wafer. In one embodiment of the invention, the desired pattern is formed on the transparent substrate by uniformly coating the transparent substrate with a reflective or dielectric layer and an opaque or blocking layer. Portions of the two layers are then sequentially removed forming a desired pattern. In another embodiment of the present invention, land portions of the reflective or dielectric layers are made undersized so that the opaque or blocking layer can be placed in contact with the transparent substrate. This reduces the thickness near the transmissive portion of the reticle.
Accordingly, it is an object of the present invention to provide a reticle which reduces thermal distortion.
It is another object of the present invention to provide a reticle that permits the use of higher illumination flux energy, thereby reducing exposure time and increasing throughput.
It is an advantage of the present invention that it is relatively easy to manufacture.
It is a further advantage of the present invention that it does not impact the use of existing photolithographic tools or equipment and processing techniques.
It is a feature of the present invention that a reflective or dielectric layer is used between the transparent substrate and the opaque or blocking layer.
It is another feature of the present invention that it can be used independent of illumination source wavelength with the appropriate reflective layer. This would include, among other wavelengths, 157, 193, 248 and 365 nanometers.
These and other objects, advantages, and features will become readily apparent in view of the following detailed description.